Duck Hook

I would like to know what causes duck hooks and how to fix them

Club face passing in front of the hands prior to impact.
Keep your hands in front and you should be able to straighten it out.

Your path is too inside out and your clubface is too closed. Fix the path first.

Bad Duck Hooks are as Greg suggests, the clubhead is whipping ahead of your hands and cutting across the ball with a closed face.

This is often caused by not making a fully committed shoulder turn, you may be swinging back and through with your arms only. Try turning the shoulders as near to 90 deg as you can in the back swing then back through with your arms and chest moving together so your hands stay in the middle of your chest.

Brian's advice is spot on; even if you're a bit "armsy" you still need to complete the backswing and NOT rush the transition. I suffered with a duck hook for ages before similar suggestions from B solved the problem. One addition though, if you're not flexible enough for a 90 degree turn then a positive pause at the top is almost as good - back, and (pause), and down or 1 and 2 and 3....